The present invention relates generally to an improvement in the recovery of bitumen from tar sands. The invention further relates to an improvement in the recovery of bitumen in water processes of extracting bitumen from tar sands. This invention particularly relates to the treatment of tailings discharged from a water extraction process and the treatment of tailings retained in a water storage retention pond. More particularly, this invention relates to a method of treating the sludge layer from a retention pond to recover bitumen froth therefrom using additional water.
Tar sands are also known as oil sands or bituminous sands. The sand deposits are found in numerous locations throughout the world, e.g., Canada, United States, Venezuela, Albania, Rumania, Malagasy and U.S.S.R. The largest deposit, and the only one of present commercial importance is in the northeast of the Province of Alberta, Canada.
Tar sand is a three-component mixture of bitumen, mineral and water. Bitumen is the component for the extraction of which tar sands are mined and processed. The bitumen content is variable, averaging 12 wt.% of the deposit, but ranging from 0 to 18 wt.%, and as used herein bitumen includes hydrocarbons. Water typically runs 3 to 6 wt.% of the mixture, increasing as bitumen content decreases. The mineral content constitutes the balance.
Several basic extraction methods have been known for many years for separating the bitumen from the sands. In a "cold-water" method, the separation is accomplished by mixing the sands with a solvent capable of dissolving the bitumen. The resulting mixture is then introduced into a large volume of water, water with a surface agent added, or a solution of a neutral salt in water. The combined mass is then subjected to a pressure or gravity separation.
The "hot water" process for primary extraction of bitumen from tar sands consists of three major process steps (a fourth step, final extraction, is used to clean up the recovered bitumen for further processing). In the first step, called conditioning, tar sand is mixed with water and heated with open steam to form a pulp of 70-85 wt.% solids. Sodium hydroxide or other reagents are added as required to maintain the pH in the range of about 8.0-8.5. In the second step, called separation, the conditioned pulp is diluted further so that settling can take place. The bulk of the sand-sized particles (greater than 325 mesh screen) rapidly settles and is withdrawn as sand tailings. Most of the bitumen rapidly floats (settles upward) to form a coherent mass known as bitumen froth which is recovered by skimming the settling vessel. An aqueous middlings layer containing some mineral and bitumen is formed between these layers. A scavenger step may be conducted in the middlings layer from the primary separation step to recover additional amounts of bitumen therefrom. This step usually comprises aerating the middlings. The froths recovered from the primary and scavenger step can be combined, diluted with naphtha and centrifuged to remove more water and residual mineral. The naphtha is then distilled for further processing. Hot water processes are described in U.S. Pat. Nos. 3,496,093; 3,502,566; 3,526,585; 3,502,565; 3,502,575; 3,951,800; 3,951,779; 3,509,641 and 3,751,358. Tailings can be collected from the aforementioned processing steps and generally will contain solids as well as dissolved chemicals. The tailings are collected in a retention pond in which additional separation occurs. The tailings can also be considered as processing water containing solids which are discharged from the extraction process. The tailings comprise water, both the natural occurring water and added water, bitumen and mineral.
The mineral particle size distribution is particularly significant to operation of the hot water process and to sludge accumulation. The terms sand, silt and clay are used in this specification as particle size designations. Sand is siliceous material which will not pass through a 325 mesh screen. Silt will pass through a 325 mesh screen but is larger than two microns and can contain siliceous material. Clay is smaller than 2 microns and also can contain siliceous material. The word fines as used herein refers to a combination of silt and clay.
Conditioning tar sands for the recovery of bitumen consists of heating the tar sand/water mixture to process temperature (180.degree.-200.degree. F.), physical mixing of the pulp to uniform composition and consistency, and the consumption (by chemical reaction) of the caustic or other added reagents. Under these conditions, bitumen is stripped from the individual sand grains and mixed into the pulp in the form of discrete droplets of a particle size on the same order as that of the sand grains. During conditioning, a large fraction of the clay particles becomes well dispersed and mixed throughout the pulp. The conditioning process which prepares bitumen for efficient recovery during the following process steps also prepares the clays to be the most difficult to deal with in the tailings disposal operation.
The other process step, called separation, is actually the bitumen recovery step, the separation having already occurred during conditioning. The conditioned tar sand pulp is screened to remove rocks and unconditionable lumps of tar sands and clay. The reject material, "screen oversize", is discarded. The screened pulp is further diluted with water to promote two settling processes. Globules of bitumen, essentially mineral-free, float upward to form a coherent mass of froth on the surface of the separation units; and, at the same time, mineral particles, particularly the sand size material, settle down and are removed from the bottom of the separation unit as sand tailings. These two settling processes take place through a medium called the middlings. The middlings consists primarily of water, bitumen particles and suspended fines.
The particular sizes and densities of the sand and of the bitumen particles are relatively fixed. The parameter which influences the settling processes most is the viscosity of the middlings. Characteristically, as the suspended material content rises above a certain threshold, which varies according to the composition of the suspended fines, viscosity rapidly achieves high values with the effect that the settling processes essentially stop. Little or no bitumen is recovered and all streams exiting the unit have about the same composition as the feed. As the feed suspended fines content increases, more water must be used in the process to maintain middlings viscosity within the operable range.
The third step of the hot water process is scavenging. The feed suspended fine content sets the process water requirement through the need to control middlings viscosity which, as noted before, is governed by the clay/water ratio. It is usually necessary to withdraw a drag stream of middlings to maintain the separation unit material balance, and this stream of middlings can be scavenged for recovery of incremental amounts of bitumen. Air flotation is an effective scavenging method for this middlings stream.
Final extraction or froth clean-up is usually accomplished by centrifugation. Froth from primary extraction is diluted with naphtha, and the diluted froth is then subjected to a two stage centrifugation. This process yields an oil product of essentially pure, but diluted, bitumen. Water and mineral and any unrecovered bitumen removed from the froth constitutes an additional tailing stream which must be disposed.
In the terminology of extractive processing, tailings are a throwaway material generated in the course of extracting the valuable material from the non-valuable material. And in tar sands processing tailings consist of the whole tar sand plus net additions of process water less only the recovered bitumen product. Tar sand tailings can be subdivided into three categories: (1) screen oversize; (2) sand tailings--the fraction that settles rapidly, and (3) middlings--the fraction that settles slowly. Screen oversize is typically collected and handled as a separate stream.
Tailings disposal is the operation required to place the tailings in a final resting place. Because the tailings contain bitumen emulsions, finely dispersed clay with poor settling characteristics and other contaminants, water pollution considerations prohibit discarding the tailings into rivers, lakes or other natural bodies. Currently the tailings are stored in retention ponds (also referred to as evaporation ponds) which involve large space requirements and the construction of expensive enclosure dikes. A portion of the water in the tailings can be recycled back into the water extraction process as an economic measure to conserve water. Currently two main operating modes for tailings disposal are (1) dike building--hydraulic conveying of tailings followed by mechanical compaction of the sand tailings fraction; and (2) overboarding--hydraulic transport with no mechanical compaction.
At one commercial location, for dike building, tailings are conveyed hydraulically to the disposal area and discharged onto the top of a sand dike which is constructed to serve as an impoundment for a pool of liquid contained inside. On the dike, sand settles rapidly and a slurry of water, silt, clay and minor amount of bitumen, as well as any chemical used during processing flows into the pond interior. The settled sand is mechanically compacted to build the dike to a higher level. The slurry which drains into the pond interior commences stratification in settling over a time scale of months to years. As a result of this long term settling, three layers form. The top layer, e.g., 5-10 feet of the pool, is a layer of relatively clear water containing minor amounts of solid, e.g., up to 5 wt.% and any dissolved chemicals. This layer of pond water can be recycled to the water extraction process without interfering with extraction of bitumen from tar sands. Below this clear water layer is a discontinuity in solid contents. Over a few feet, solids content increases to about 10-15 wt.% and thereafter, solids contents increase regularly toward the pond bottom. In the deeper parts of the pond, solid contents of over 50 wt.% have been measured. This second layer is commonly called the sludge layer. In general the sludge layer can be characterized as having more than 10 wt.% of solids (which is defined as mineral plus bitumen). More particularly, the sludge can be characterized as having 20 wt.% to 50 wt.% solids. Also the sludge can be characterized as having about 0.5 to about 20 wt.% bitumen. The solids contents of the sludge layer increase regularly from top to bottom by a factor of about 4-5. Portions of the solids are clays. The clays, dispersed by the processing, apparently have partially reflocculated into a fragile gel network. Through this gel, particles of larger-than-clay sizes are slowly settling. Generally this sludge layer cannot be recycled to the separation step because no additional bitumen is extracted. A third layer formed of sand also exists.
Overboarding is the operation in which tailings are discharged over the top of the sand dike directly into the liquid pool. A rapid and slow settling process occurs but this distinction is not as sharp as in the previously described dike building and no mechanical compaction is carried out. The sand portion of the tailings settles rapidly to form a gently sloping beach, extending from the discharge point toward the pond interior. As the sand settles, a slurry drains into the pool and commences long-term settling. Thus water in ponds prepared by both dike building and overboarding can be included in the general definition of sludge in the present description.
Methods for treating sludge formed in a retention pond used to store tailings from a hot water extraction of bitumen from tar sands are disclosed in Canadian Pat. Nos. 975,696; 975,697; 975,698; 975,699 and 975,700. The first mentioned Canadian Patent discloses removing sludge from a pond, placing the sludge in an air scavenger treating zone wherein the sludge is aerated and agitated concurrently, to form an upper bitumen froth layer and a lower tailings of water and mineral water. The lower tailings can be discharged into a retention pond. The upper bitumen froth is sent to a settling zone wherein two layers are formed, an upper bitumen layer reduced in mineral matter and water and a lower layer comprised substantially of mineral matter and water with minor amounts of bitumen. The latter lower layer is recycled back to the air scavenger treating zone while the upper bitumen layer is processed further to recover the bitumen. This Canadian patent and the others also disclose that sodium silicate can improve bitumen recovery when used in connection with the aeration and agitation. Canadian Pat. No. 975,697 discloses a process similar to that described in the previous patent with an additional step in that a portion of the lower layer, which otherwise would be recycled back to the air scavenger treating zone, is returned to the retention pond. Canadian Pat. No. 975,698 discloses feeding the sludge from a retention pond to an air pressure zone wherein the sludge is aerated at superatmospheric pressure to aerate bitumen in the sludge. Canadian Pat. No. 975,699 discloses feeding sludge recovered from a retention pond to a settling zone and permitting the sludge to form an upper froth layer and a lower tailings layer. Canadian Pat. No. 975,700 discloses feeding sludge to an air scavenger treating zone wherein the sludge is aerated and agitated concurrently and resulting froth is separated in the scavenger treating zone while the tailings are returned to the pond. However, none of the foregoing Canadian patents disclose or suggest diluting the sludge with water containing less than substantial amounts of mineral matter prior to aeration and agitation.
U.S. Pat. No. 4,018,664 discloses a method for treating sludge from a retention pond associated with hot water extraction of bitumen from bitumen sands. The method involves withdrawing sludge from a pond, diluting and mixing it with water, and settling to obtain a froth layer, a middle layer containing reduced solids and a lower layer containing increased solids over the original sludge. Agitation and/or aeration, for example, aeration sufficient to mildly agitate the sludge, are disclosed as beneficial and essential to the extent that proper mixing is achieved. Proper mixing presumably means that the sludge and dilution water are in such close association that samples taken anywhere in the mixture all would contain essentially the same amount of water. In contrast, as used herein, high-shear agitation refers to a degree of turbulence significantly greater than that required for mere mixing.